Enhanced accumulation of root hydrogen peroxide is associated with reduced antioxidant enzymes under isoosmotic NaCl and Na₂SO₄ salinities

Abstract

The inhibitory effect of salt stress on lettuce is one of the main reasons for the reduction of plant growth and crop productivity. In the present study, the response of two lettuce varieties Verte and Romaine to isoosmotic NaCl and Na₂SO₄ treatments were examined. Both varieties were grown in pots containing nutrient Hoagland solution with or without 100 mM NaCl or 77 mM Na₂SO₄. Relative growth rate (RGR), hydraulic parameters, root ion content, proline and several antioxidant activities in roots were measured after 12 days of treatment. After prolonged exposure to salt stress, relative growth rate and water content of lettuce significantly decreased. Roots accumulated high level of Na⁺ under both salts, whereas the accumulation of K⁺ and Ca²⁺ decreased. High level of Na⁺ inside the cells inhibited the K⁺ uptake and resulted in increased K⁺/Na⁺ ratio. In addition, salt stress also caused an increase in the accumulation of proline. This result suggests that proline may play a crucial role in protecting lettuce under salt stress especially in response to Na₂SO₄ treatment. Membrane damage estimated by electrolyte leakage (EL) increased especially in response to Na₂SO₄ treatment in both varieties, but Verte had significantly lower EL relative to Romaine under 100 mM NaCl. A reduction in the activities of CAT in both varieties under 100 mM, and GPX activity in Verte under Na₂SO₄ treatment coincided with an increase in H₂O₂ level, indicative of cellular damage and a general depression of the antioxidant enzymatic system in lettuce roots.

Keywords: Lettuce, NaCl, Na₂SO₄, RGR, mineral nutrition, antioxidant activities, proline